Tracers are used to follow fluid flow in various systems such as oil reservoirs, process flow lines, ground water is leakage studies etc. Tracers are released up-stream and detected downstream. Analysing tracer concentration downstream may give information about flow rates, dilution volumes, communication, fluid mixing, mass residence time distribution etc. Tracers may be injected into the system by a variety of methods, however the most common being either as a sharp pulse (Dirac pulse) or at a constant concentration rate. In several types of well operation there is a need for tracer delivery systems, which are able to release tracers at places where it will, due to practical constrains, be difficult to position or install more conventional mechanical injection equipment. There is also a need for delivery systems that are able to reflect changes in the well conditions such as temperature, single-phase fluid chemical composition or fluid phase types.
Examples of suitable non-radioactive tracers are salts of naphthalenesulfonic acids, salts of amino naphthalenesulfonic acids, fluorescein and fluorinated benzoic acids. 3H-labelled or 14C-labelled tracers of the same kind of components may also be applied.
Application of such release methods has been proposed in several patents with the specific aim to measure fluid inflow in oil production wells (patent US06645769, patent US0582147). Due to the complexity of most oil reservoirs and the modern horizontal, undulating, multilateral or multi-branched production wells it may be difficult to know from which well or zones in the well the fluids are produced. The water production rates in oilfields may in some wells be at a level of 10.000-20.000 m3/d for many years. The tracer release system has to be able to deliver tracer amounts giving a tracer concentration above the detection limit at the downstream detection position. In many situations the available and accessible volume for the delivery system is limited. The system should therefore be able to deliver tracers that are detectable in sub ppb concentrations. This is easily achieved with radioactive tracers (mainly pure beta-emitters). In many systems, however, this should be avoided and the radioactive tracers should be replaced with non-radioactive chemical species.
Optimal oil production from the reservoir depends upon reliable knowledge of the reservoir characteristics. Traditional methods for reservoir monitoring include seismic log interpretation, well pressure testing, production fluid analysis, production history matching and interwell tracer techniques. Due to the complexity of the reservoir all information available is valuable in order to give the operator the best possible knowledge about the dynamics in the reservoir. One common secondary oil recovery process is water injection in dedicated injection wells. The water may travel in different layers and sweep different area in the reservoir. Monitoring of the production of this water in different zones in the well is important to design a production program that improves the sweep efficiency and thereby increased the oil recovery. Mixing of injection water and formation water originally present in the reservoir may cause supersaturated solutions leading to precipitation of particles (scale) in either the reservoir near-well zone or in the production tubing. By knowing which zones contribute to water production, action can be taken to reduce the effect of scaling and thereby maintain productivity.